Haven't seen this response yet, so here's what I do: cardboard (big sheets from refrigerator boxes and such that I get free from the local True Vaule store) but instead of one layer, I put down two. That way, when the top layer gets so nasty I can't stand it any more (or when it starts wearing through, like somebody said earlier,) I can just take up that layer and still have another layer left to finish the season with.

The double layer also adds quite a bit of welcome cushioning for old tired feet on my concrete floor.